Bio::DB::GFF::Aggregator::clone is one of the default aggregators, and was written to be compatible with the C elegans GFF files. It aggregates raw "Clone_left_end", "Clone_right_end", and "region:Genomic_canonical" features into composite features of type "clone".

The WormBase GFF model is unusual in that clones aren't identified as a single feature with start and stop positions, but as two features, a "left end" and a "right end". One or both of these features may be absent. In order to accommodate this, the aggregator will return undef for the start and/or stop if one or both of the ends are missing.